1. Field of the Invention
The present invention relates to an autotensioner for keeping the tension of a belt of a belt drive mechanism automatically to a proper level.
2. Description of the Related Art
In the belt drive mechanism of the prior art in which the belt is made to run on a drive pulley and a driven pulley, there is used an autotensioner as means for applying a tension to the belt. This autotensioner generally includes a tension pulley for applying a tension to the belt; a support arm for supporting the tension pulley; a torsion coil spring for biasing the support arm in one direction; and a friction member for establishing a resistance (or sliding resistance) to the rocking motions of the support arm. In this autotensioner, the motions of the tension pulley are regulated, while being allowed according to the fluctuations in the tension of the belt, so that the support arm may be elastically biased in one rocking direction by the torsional restoring force of the coil spring and given a rocking resistance by the friction member.
In this autotensioner, as disclosed in Unexamined Published Japanese Patent Application No. 2-253035, the friction member is made of an annular plate for realizing the function of attenuating vibrations and impacts. The friction member is sandwiched between the front and end face of the boss or the component of the support arm and a guide member, as mounted on the front end of a rocking support pin for supporting the support arm, while employing them as its sliding contact partners. Moreover, the friction member is pressed between the sliding contacting partners by the extendible restoring force thereby imparting the rocking resistance to the support arm by its friction.
The friction member is usually made of a relatively hard material such as a material having a Rockwell hardness [H.sub.R R] of about 50 or more. The material for the friction member is prepared by adding glass fibers to the base of either a thermoplastic synthetic resin such as polyamide (PA), polyether ketone (PEEK) or polyether sulfone (PES), or a thermoset synthetic resin of phenol.
In the autotensioner described above, the friction member is made of a relatively hard material mainly with an object of suppressing an abrasion. When the sliding contact partners have a large manufacturing distortion, the friction member has a region, where it is out of contact with the partners, at the initial state of use. As the abrasion of the friction member proceeds according to the use so that the non-contact region is lessened, the friction coefficient of the friction member to the sliding contact partners seriously changes. This invites an instability in the vibration/impact attenuation characteristics of the friction member.
When the friction member is made of a relatively soft material, on the contrary, it can come into close contact with even the sliding contact partners having the manufacturing distortion. It is then expected that the change in the friction coefficient between the initial and advanced stages can be suppressed to a small value. However, the friction member has a short lifetime because of less caused by abrasion.